Since the first widespread introduction of home electronics, efforts have been made to make home entertainment systems closer to live entertainment, or to commercial movie theaters. Among other improvements, efforts have been made to increase the number of sound channels to enhance the home-theater experience to produce more enveloping and convincing sound reproduction. This trend has been accelerated in no small part by the advent of digital signal transmission and storage, which has widely increased the options and alternatives available.
A recent standard for digital audio is known as AC-3, promulgated by Dolby Laboratories and currently anticipated for wide use in connection with digital television and audio transmissions, as well as digital storage media. The AC-3 standard provides for delivery, from storage or broadcast, of up to six channels of audio information, specifically, left, right and center channels, as well as left surround, right surround, and low frequency effect channels. Further information on the AC-3 standard can be found in "Digital Audio Compression (AC-3) Standard", published by the United States Advanced Television Systems Committee, Dec. 20, 1995, and C. Topp et al., "AC-3: Flexible Perceptual Coding for Audio Transmission and Storage", AES 96.sup.th Convention (February 1994).
Although the AC-3 standard allows for up to five channels of wideband audio information, plus a single channel of low frequency effects, in many cases a given audio program may include fewer than five wideband and one low frequency channel. For example, a typical older stereo program may include only left and right channels. The AC-3 standard provides for such situations by defining 8 different audio coding modes, known as "ac-modes" in which the five wideband channels may be stored or transmitted compatibly with the AC-3 standard. (In addition, the digitally stored or transmitted program may, or may not, further include a sixth low frequency channel.) The number and nature of the wideband channels provided by seven of the eight ac-modes, are described in the following table:
______________________________________ ac-mode channels wideband channel descriptions ______________________________________ 1 1 Center 2 2 Left, Right 3 3 Left, Center, Right 4 3 Left, Right, Surround 5 4 Left, Center, Right, Surround 6 4 Left, Right, Left Surround, Right Surround 7 5 Left, Center, Right, Left Surround, Right Surround ______________________________________
In addition to the seven input modes identified in the preceding table, there is also an eighth audio coding mode, known as ac-mode0. When audio is received in ac-mode0, special output formats may be invoked, as discussed in detail below.
The number of channels that can be reproduced at a particular installation will vary. Because many sound systems are not equipped with a full complement of speakers capable of delivering the channels that may be encoded under AC-3, the channels provided by an AC-3 formatted signal must be "downmixed" for delivery via fewer than a full complement of speakers.
Specifically, when the input signal to an AC-3 compatible sound system uses one of ac-modes 1-7 identified by the above table, the output signal may be produced in one of eight output modes, known as "output.sub.-- modes". The eight output.sub.-- modes, and the number and nature of the channels produced under each mode, are described in the following table:
______________________________________ output.sub.-- mode channels channel descriptions ______________________________________ 2/0 2 Left, Right 1/0 1 Center 2/0 2 Left, Right 3/0 3 Left, Center, Right 2/1 3 Left, Right, Surround 3/1 4 Left, Center, Right, Surround 2/2 4 Left, Right, Left Surround, Right Surround 3/2 5 Left, Center, Right, Left Surround, Right Surround ______________________________________
In addition to these output modes, as noted above, special output modes are available when an input signal is delivered in ac-mode 0. Specifically, when the input is delivered in ac-mode 0, the output format is selected by identifying (a.) the number of front speakers (1, 2 or 3), whether the output should be in a stereo format (DUAL.sub.-- STEREO), a monophonic format derived from the left channel (DUAL.sub.-- LEFTMONO), a monophonic format derived from the right channel (DUAL.sub.-- RIGHTMONO), or a monophonic format derived from a mixture of both stereo channels (DUAL.sub.-- MIXMONO).
For each combination of an input mode (ac-mode value) and an output mode (output.sub.-- mode value or, in the case of ac-mode 0, number of front speakers, and STEREO/MONO settings, as described above), the output channels are generated by collecting samples from the wideband input channels into a five-dimensional vector i, and premultiplying the vector i by a 5.times.5 downmixing matrix D, to form a resultant five-dimensional vector o containing the corresponding samples of the output channels. Specifically, the downmixing equation is: EQU o=D.multidot.i
Where i is a five-dimensional vector formed of samples from the Left, Center, Right, Left Surround and Right Surround input channels, i.sub.L, i.sub.C, i.sub.R, i.sub.LS, i.sub.RS, respectively: ##EQU1## o is a five-dimensional vector formed of corresponding samples from the Left, Center, Right, Left Surround and Right Surround output channels, O.sub.L, O.sub.C, O.sub.R, O.sub.LS, O.sub.RS, respectively: ##EQU2## and D is a 5.times.5 matrix of downmixing coefficients: ##EQU3## The reader will appreciate that this matrix computation involves multiplying each of the coefficients d.sub.** in the downmixing matrix D by one of the input channel samples to form a product. These products are then accumulated to form samples of the output channels.
Various values of coefficients d.sub.** in the downmixing matrix D are used for downmixing in each of the 71 possible combinations of input and output modes supported by AC-3. In some cases, the downmixing coefficients d.sub.** are computed from parameters stored or broadcast with the AC-3 compliant digital audio data, or parameters input by the listener. The appendix to this application describes the values of the coefficients in downmixing matrix D, for each of the 71 permitted combinations of input and output modes, for reference.